Solid state plasma antenna

ABSTRACT

A solid state electronically steerable antenna can be generated from a sheet of semiconductor material by forming a pattern of localised plasma regions in the sheet, either by injecting carriers into, or by generating carriers in, those localised regions. A suitable solid state plasma antenna can be made from a silicon wafer ( 10 ) by first thermally oxidising the surfaces and subjecting the wafer ( 10 ) to a high temperature stabilisation process to improve the stoichiometry at the silicon/silica interface, and optionally also performing a low-temperature bake in a gas mixture including hydrogen. This produces a wafer ( 10 ) with a long minority carrier lifetime. Regions of the wafer ( 10 ) in which plasma may be generated are (hen defined by reticulation to form isolated regions with high minority carrier lifetime. The resulting discrete regions may be of a size less than 1 mm, for example 0.2 mm.

This invention relates to a solid state antenna, and to a process forits manufacture.

In the field of wireless communications, there is a desire to operate athigher frequencies, for example greater than 1 GHZ. For this purpose, itwould be desirable to develop a solid state, electronically steerableantenna. One way in which this may be achieved is to form a sheet ofsemiconductor material with a-pattern of electrically conducting regionson its opposed surfaces, and to generate conducting plasma elements ofcharge carriers within the semiconductor material to coupleelectromagnetic radiation to or from the antenna, and to generate apattern of such conductive elements to reflect or absorb theelectromagnetic radiation. Such localised plasma elements may be createdby illuminating that part of the semiconductor sheet with suitableradiation (for example infrared or visible light) of photon energygreater than the band gap (which for silicon is about 1.1 eV), or byinjecting charge carriers. The solid state antenna may be, for example,that described in Patent Nos. PCT/GB01/02813 or PCT/GB02/01925.

A crucial factor in determining the power required to create and sustainsuch a plasma is the lifetime of the minority carrier in thesemiconductor. The higher the lifetime, then the lower is the power. Itis possible to obtain silicon in bulk, in which the lifetime is greaterthan 10 ms. However, on an untreated wafer, the surface contains a highdensity of dangling bonds and other electronic defects which reduce theeffective lifetime to between 10 and 100 μs. The surface effects can beconsiderably reduced by thermal oxidation to passivate the siliconsurface. There will still be defects at the silicon silica interface,but these can be minimized by subsequent treatment.

According to a non-limiting embodiment of the present invention there isprovided a method of forming a solid state plasma antenna, which methodcomprises:

-   -   (a) selecting a semiconductor wafer,    -   (b) subjecting surfaces of the wafer to thermal oxidation,    -   (c) subjecting the wafer to stabilisation in a gas mixture        incorporating a minor proportion of oxygen at a temperature        above 800° C. to improve the stoichiometry at a silicon/silica        interface,    -   (d) and, optionally, performing a low-temperature bake in a gas        mixture including hydrogen at a temperature above 300° C. to        reduce interface state density;        -   and then localising regions of the wafer in which plasma may            be generated by reticulation to form a network of isolated            regions with high minority carrier lifetime, by one or more            of the following steps:    -   (e1) selectively removing the layer developed by steps (b),        (c), (d) by etching, scoring, abrading or ablation,    -   (e2) partially or fully cutting through the wafer, for example        using an anisotropic etch, a saw, a plasma etch, an ablation        technique, or a laser,    -   (e3) depositing a metal grid onto the silica surface,    -   (e4) effecting local deposition and diffusion or implantation of        a dopant such as boron or phosphorus, and    -   (e5) effecting implantation of hydrogen, helium or gold ions.

Steps (b) and (c), and step (d) when present, may be repeated, forexample after step (e).

Preferably in step (c) the gas mixture is predominantly of anon-reactive gas such as nitrogen, and the proportion of oxygen is lessthan 20%, by volume, for example 10% by volume. If step (d) is adopted,preferably the gas mixture incorporates a non-reactive gas such asnitrogen, and may be a mixture of equal volumes of nitrogen andhydrogen.

The method of the invention may be one wherein the cut is performed byan anistropic etch, a saw, a plasma etch, an ablation technique, or alaser.

Preferably the semiconductor is silicon. The isolated regions may be ofa size of less than 1 mm. The isolated regions may form an arraycovering an area of the wafer.

A plasma may be generated at a selection of the isolated regions in thearray, the selection being such as to focus radiation at a desiredposition. For example, the selected regions may be illuminated withinfrared radiation so as to create an electron-hole plasma.Alternatively an array of PIN diodes may be formed on the surface orthrough the thickness, and may be selectively forward biased to createthe desired plasma.

The invention also extends to a solid state antenna made by the methodof the invention.

The invention will now be further and more particularly described, byway of example only, and with reference to FIG. 1, which shows a planview of part of a solid state antenna.

The solid state antenna consists of a circular silicon wafer 10, ofdiameter 135 mm and of thickness 300 microns. The wafer 10 is made of ahigh quality pure silicon. The wafer 10 is subjected to thermaloxidation in an atmosphere containing oxygen, so a layer of silicondioxide (silica) is formed over its entire surface. The wafer 10 is thensubjected to a stabilisation procedure in the nitrogen atmospherecontaining 10% oxygen (by volume) at a temperature of above 900° C.(e.g. 950° C.), the wafer being held in this temperature for an hour.The wafer 10 is then subjected to a bake procedure at 450° C. in anatmosphere of a nitrogen/hydrogen mixture, to reduce interface statedensity. The resulting wafer 10 has substantially uniform properties,and a long minority carrier lifetime, typically about 5 ms.

The upper and lower surfaces of the wafer 10 are then masked so as todefine, on each surface, an identical square grid or network of lines 12each of width of 5 μm defining squares 14 between the lines, each square14 having sides of 200 μm. The wafer 10 is then subjected to an aqueousetching process in which the oxide layer is removed by etching from thatgrid or network of lines 12. Consequently the wafer 10 is subdividedinto an array of square regions 14 in which the minority carrierlifetime is high, separated by the grid 12 in which the minority carrierlifetime is comparatively short.

Optical fibres (not shown) are then coupled to the upper surface of thewafer 10 so that radiation of an appropriate wavelength can betransmitted to each of the square regions. Alternatively the radiationmay be supplied to the square regions 14 from a source such as a diodearray or a flat screen display. If radiation is supplied to one suchsquare region 14, of sufficient photon energy to generate chargecarriers and at sufficient intensity, then in that region 14 there iscreated an electrically conducting plasma. Hence by supplying radiationto an array of such square regions 14, an electrically conducting regionof the wafer 10 is formed, and the antenna is able to be electronicallysteerable. The array may be, for example, a straight line, so creating astraight line conducting region which will act as a plane mirror forincident microwaves (because the wavelength of the microwaves is muchgreater than the size of the discrete regions 14). Such a straight linemirror can be arranged so that radiation incident in the plane of thewafer 10 is focused at the centre of the wafer 10, and there may be anelectrical feed or contact at the centre, for example an embedded pin.

It is to be appreciated that the embodiment of the invention describedabove with reference to the drawing has been given by way of exampleonly and that modifications may be effected. Thus, for example, ratherthan having the grid of lines 12 covering the entire upper and lowersurfaces of the wafer 10, the grid may instead cover only a part of thesurface, for example a circular region of diameter 60 mm around thecentre of the wafer 10. The wafer 10 may be of different dimensions, forexample of a diameter in the range 15 mm up to 200 mm, more typically upto 150 mm; and of thickness in the range 0.1 mm up to 10 mm, preferablybetween 0.1 mm and 5 mm. The size of the discrete regions 14 may bedifferent from that described above, as long as it is much less than thewavelength of the radiation to be transmitted or received by theantenna. Indeed the discrete regions might be of a different shape, forexample rectangular rather than square. The discrete regions may defineone or more lines, rather than covering an area. A range of differenttreatments may be adopted to reduce the minority carrier lifetime alongthe lines 12 on the wafer 10.

1. A method of forming a solid state plasma antenna, the methodcomprising: (a) selecting a semiconductor wafer, (b) subjecting surfacesof the wafer to thermal oxidation, (c) subjecting the wafer tostabilisation in a gas mixture incorporating a minor proportion ofoxygen at a temperature above 800° C. to improve the stoichiometry at asilicon/silica interface, (d) and, optionally, performing alow-temperature bake in a gas mixture including hydrogen at atemperature above 300° C. to reduce interface state density; and thenlocalising regions of the wafer in which plasma may be generated byreticulation to form a network of isolated regions with high minoritycarrier lifetime, by one or more of the following steps: (e1)selectively removing the layer developed by steps (b), (c), and (d) beetching, scoring, abrading or ablation, (e2) partially or fully cuttingthrough the wafer, (e3) depositing a metal grid onto the silica surface,(e4) effecting local deposition and diffusion or implantation of adopant, and (e5) effecting implantation of hydrogen, helium or goldions.
 2. A method as claimed in claim 1 wherein steps (b) and (c) arerepeated, and wherein step (d) is also repeated when step (d) ispresent.
 3. A method as claimed in claim 1 wherein in step (c) the gasmixture is predominantly of a non-reactive gas such as nitrogen, and theproportion of oxygen is less than 20% by volume.
 4. A method as claimedin claim 1 including the step (d), wherein in step (d) the gas mixtureincorporates a non-reactive gas.
 5. A method as claimed in claim 4 inwhich the non-reactive gas is nitrogen.
 6. A method as claimed in claim5 in which the non-reactive gas is a mixture of equal volumes ofnitrogen and hydrogen.
 7. A method as claimed in claim 1 wherein the cutis performed by an anistropic etch, a saw, a plasma etch, an ablationtechnique, or a laser.
 8. A method as claimed in claim 1 wherein in step(e4) the dopant is boron or phosphorus.
 9. A method as claimed in claim1 wherein the semiconductor is silicon.
 10. A method as claimed in claim1 wherein the isolated regions are of size less than 1 mm.
 11. A methodas claimed in claim 1 wherein the isolated regions form an arraycovering an area of the wafer.
 12. A solid state antenna made by amethod as claimed in claim 1.